198 FAMILY: AMCEBIDiE 



change, as can easily be noted by observing living amoebse in which the 

 nucleus can be seen. Owing to the density of the cytoplasm and its high 

 refractive index, the delicate nucleus of E. histolytica is often difficult to 

 detect in the living amoebae. The nucleus of E. coli, on account of the less 

 dense cytoplasm, is more readily seen, while structurally it is very different 

 from that of E. histolytica. 



E. histolytica mulitiplies in the tissues by binary fission. There is 

 first a division of the nucleus, the details of which have been described by 

 Dobell (1919), and this is followed by division of the cytoplasm into two 

 more or less equal parts. Kofoid and Swezy (1924a, 1925) state that there 

 are six chromosomes which appear during nuclear division (Fig. 57). 

 Reproduction by bud formation, as described by Schaudinn (1903), and 

 by a process of schizogony, as recorded by Job and Hirtzmann (1918), 

 are undoubtedly the result of observations on degenerate amoebse, or even 

 tissue cells. On one occasion the writer has observed amoebae with two 

 and four nuclei in liver-abscess pus (Fig. 95, 6). 



2. Precystic Forms. — As already explained above, under certain con- 

 ditions E. histolytica becomes encysted, and as the cysts are smaller than 

 the tissue-invading amoebae, it is evident that before encystment smaller 

 amoebae are produced. These are probably developed from the large 

 amoebae by division, while the daughter amoebae, instead of increasing in 

 size as they do in the tissues, divide again, so that increasingly small forms 

 are produced. It is possible that the large amoebae, which have become 

 more superficial in position in the intestinal lesions, suffer from a lack of 

 fresh tissue or fluid nutriment on which to feed, so that after division 

 growth does not take place. This shortage of food may be the stimulus 

 which leads to encystment. The size of the amoebae which actually 

 encyst varies considerably, and evidence has been brought forward by 

 the writer and O'Connor (1917), and by Dobell and Jepps (1917, 1918), 

 that there exist definite races of E. histolytica which can be distinguished 

 from one another by the average size of the cysts. In the races with small 

 cysts these may have an average diameter of 7 microns only, while in 

 those with larger cysts it may be as much as 18 microns. It follows, 

 therefore, that the precystic amoebae may vary in diameter from 7 microns 

 upwards (Fig. 95, 7-11). There does not appear to be any evidence to 

 support the view that in those races with small precystic amoebae the 

 corresponding tissue-invading forms are smaller than in those producing 

 larger precystic amoebae. 



The precystic amoebae have the same general structure as the tissue- 

 invading forms, but the cytoplasm is devoid of food vacuoles, the amoebae 

 having ceased to ingest red blood-corpuscles or other cells, and having 

 got rid of the remains of those taken in previously. 



